Measurements of gaseous species in air are of considerable interest for applications such as environmental monitoring etc. However, it can be difficult to perform such measurements accurately and inexpensively. Most emissions sources will result in a plume of the species of interest being emitted into the atmosphere, with the direction and spread of the plume being determined in complex and highly variable manner by the (turbulent) transport of the atmosphere. A single-point measurement is not likely to provide an accurate estimate of the total emission in the plume, and multiple sequential measurements at different locations are subject to errors caused by motion of the plume (e.g., as local wind conditions vary). Simultaneous measurements using an analyzer at each measurement location can address this issue, but this approach has the disadvantage of being costly (i.e., expensive analyzer hardware is duplicated).